1. Field of the Invention
This invention relates to a process for the production of alkaline earth metal hydroxides, particularly for the electrochemical production of barium hydroxide utilizing ion exchange membrane technology.
2. Background of the Invention
Barium hydroxide is typically produced commercially using one of a number of chemical processes. One example is that disclosed in German Patent No. 519,891 (Oct. 22, 1929) whereby barium sulfide in water solution is contacted with air, oxygen therein reacting with barium sulfide to form a mixed barium polysulfide/barium hydroxide solution from which barium hydroxide is crystallized. Other methods are known, for example that disclosed in U.S. Pat. No. 3,366,449 to Chemical Products Corporation in which an aqueous solution of barium sulfide is passed through a column of internally bifunctional ion exchange resin followed by elutriation with water to recover a dilute solution of barium hydroxide. Another example is disclosed in U.S. Pat. No. 1,136,133 in which barium hydroxide is crystallized from a solution of barium sulfide or chloride enriched in hydroxyl ions by the addition of either sodium hydroxide or ammonium hydroxide.
These methods, as well as others that could be cited, fail to be commercially attractive because of low chemical conversion of the raw materials to the desired product, excessively high impurity levels, environmental constraints, or combinations thereof.
The possibility of making barium hydroxide via electrochemical means was investigated as early as 1903 (Compt.Rend. 1903, v135, p1195). Following that, several patents teach the electrolytic treatment of barium chloride to make barium hydroxide, e.g. A. Clemm U.S. Pat. No. 973,171 (1910); German Patent 227,096 (1907); British Patent 5,471 (1910), all utilizing a diaphragm divided electrochemical cell. Later publications describe the use of mercury cell to produce barium hydroxide (P. P. Fedotiev, Z. Anorg. Chem., 1914, 86, 325; L. A. Isarov, Tr. Nauch.-Issled. Inst. Osn. Khim, 1969, 155. More recent Russian work based on a diaphragm cell process was disclosed in USSR Patent 361,143 (1972) and published in Pereab. Margants Polimeta, Rud Gruz., 1974. Another example describing the electrolysis in a diaphragm cell of barium sulfide to provide barium hydroxide is reported in Brazil Patent 85 05275 (1987).
All of these electrochemical methods disclosed to date have involved either mercury cell technology which is environmentally unacceptable for new processes today or diaphragm cell technology which results in excessive impurities, e.g. chloride, in the product. These processes are technically similar to processes which today are widely used in the chloralkali industries for the production of sodium hydroxide, chlorine and hydrogen by the electrolysis of sodium chloride brine. Another process widely used in the chloralkali industry to overcome some of the above objections to the mercury cell and diaphragm cell processes is known as the ion exchange membrane process, or membrane process for short.
The membrane process is similar to the diaphragm cell process in that a 2-compartment cell is created, thus separating the cathode from the anode. In the membrane process the separator is a non-porous sheet of material having the ability to transfer ions, preferably cations, from the anode compartment into the cathode compartment under the influence of an imposed electrical field. Electrical neutrality is maintained by means of the electrochemical processes that operate at the anode and cathode. The membrane process is used world wide today in the production of millions of tons per year of sodium hydroxide, chlorine and co-product hydrogen. Important to the success of such processes is the selection of the type of membrane used in the cell. Membranes which have been found to be particularly successful are characterized as having a bilayer structure, in which the major structure is a perfluorinated sulfonated polymer. The surface in contact with the catholyte is a perfluorinated carboxylate polymer. However, the use of membranes in the chloralkali industry requires that the feed brine be exceptionally pure with respect to alkaline earth cations, e.g. magnesium, calcium, strontium, and barium The presence of alkaline earth cations in the brine feed to the cell leads to degradation of membrane performance and can cause the membrane to rupture. Thus, it is widely believed in the art that the detrimental effect of alkaline earth cations on the membrane would preclude the direct electrolysis of alkaline earth brine to the corresponding hydroxide.
The present invention provides a process and apparatus for the production of alkaline earth metal hydroxide of exceptionally high purity by the electrolysis of alkaline earth metal halide brine in a membrane-divided electrochemical cell, the membrane being typified as a perfluorinated, sulfonated cation exchange membrane, thereby overcoming the objectionable features of all other proposed chemical and electrochemical processes.
The novel process disclosed herein is an electrochemical process in which alkaline earth metal halide brine (the anolyte) is circulated through the anode half-cell of an electrolysis cell. The cathode half-cell contains a circulating stream of alkaline earth metal hydroxide (the catholyte). The two half-cells are separated by a perfluorosulfonate polymer xe2x80x9cion exchangexe2x80x9d membrane which is chemically capable of allowing the alkaline earth metal ions to transfer from the anolyte to the catholyte under the influence of the imposed electrical potential difference between the cathode and anode, thereby completing the electrical circuit by the transfer of ions across the membrane.
In the preferred embodiment of the process, the alkaline earth metal hydroxide is barium hydroxide, and the preferred alkaline earth metal halide is barium chloride. Surprisingly, it was found that barium ions do not significantly degrade the performance of the membrane in this process. The quantity of barium ions so transported is equivalent chemically to the quantity of hydroxide ions produced by the cathode-side electrolysis of water. The main reactions occurring in the process are:
There are also several reactions that may occur at the anode, resulting in inefficiencies in the run; the primary ones are:
Thus, the catholyte solution concentration of barium hydroxide increases with elapsed time of electrolysis unless the catholyte is diluted or barium hydroxide is removed from the catholyte solution by crystallization or by other means.